In other words, some of the HTML I'm editing has a cell, with text, and then additional text on the next line (after a break). I'm trying to remove the newline, move the 2nd line of text up to the first.

matches "xxx" followed by any number of characters followed by "yyy" followed by any number of characters followed by "zzz". The parentheses mark a "capture group"; the first set of parentheses is capture group number 1, and the second set of parentheses is capture group number 2. You can have pretty much however many capture groups you want, and you can have capture groups inside capture groups. The number of a group is determined by counting left parentheses from the beginning of the regular expression.

In your replacement text, you use $n to refer to the text captured by the nth capture group. So